This invention is in the field of disk drive systems, and is more specifically directed to control of a voice coil motor in retracting the actuator in a disk drive system.
Magnetic disk drive technology is the predominant mass non-volatile storage technology in modern personal computer systems, and continues to be an important storage technology for mass storage applications in other devices, such as portable digital audio players. As is fundamental in the field of magnetic disk drives, data is written by magnetizing a location (“domain”) of a layer of ferromagnetic material disposed at the surface of a disk platter. Each magnetized domain forms a magnetic dipole, with the stored data value corresponding to the orientation of that dipole. The “writing” of a data bit to a domain is typically accomplished by applying a current to a small electromagnet coil disposed physically near the magnetic disk, with the polarity of the current through the coil determining the orientation of the induced magnetic dipole, and thus the data state written to the disk. In modern disk drives, a magneto-resistive element is used to sense the orientation of the magnetic dipole at selected locations of the disk surface, thus reading the stored data state. Typically, the write coil and the magneto-resistive element are physically placed within a read/write “head”.
In conventional disk drive systems, a spindle motor rotates the disk platters, and a “voice coil” motor moves an actuator arm on which the read/write heads are mounted, at a distal end from the motor. The voice coil motor thus moves the read/write heads to the track of the disk surface corresponding to the desired address. As known in the art, the read/write heads are physically very close to, but do not touch, the surface of the magnetic disks. In modern disk drives, the read/write heads are disposed within a “slider” at the distal end of a head gimbal assembly (HGA) suspension. The flexible HGA suspension is attached to the actuator arm, which as mentioned above is positioned by the voice coil motor. The relative motion between the spinning disk surface and the slider creates a lifting force on the slider, establishing an air bearing surface (ABS) on which the slider rides over the disk surface. The “fly height” of the heads over the disk surface thus results from the aerodynamics of the heads relative to the spinning disks, and is typically controlled by conducting a current through a resistor in the slider, so that thermal expansion of the heads determines the desired fly height. For maximum data density, the fly height is preferably as low as possible. On the other hand, relatively small asperities in the disk surface can cause contact between the slider and the disk surface at extremely low fly heights, such contact resulting in wear on both the slider and the disk surface and causing contamination from wear particles. In some cases, the heads may stick at locations of the disk surface where contact is made.
In conventional modern disk drive systems, the read/write heads are “parked” during such time that the disk drive is not in operation. Typically, this “parking” involves the voice coil motor positioning the actuator at a parking position at the inner or outer limit of the disk radius. Usually, the parking positions are locations of the disk surface that do not store data, and that are textured so that the heads do not stick to the disk surfaces, considering that the heads will collapse to the disk surface once the disks stop spinning. In general, as is well known in the art, the parking position of the actuator and the read/write heads, upon shutdown, includes a wedge-shaped ramp that the heads at the actuator arm contact and “climb”, the top surface of which safely supports the heads at shutdown, in the absence of lift from the spinning disk platters. In addition, a “crash stop” is typically positioned past the ramp and parking position, to prevent the actuator arm from moving past the parking position, and to absorb excess kinetic energy from the actuator.
To avoid damage to the disk surface, modern disk drives typically include some provision for parking the read/write heads in the event of a sudden loss of power supply voltage. This retracting of the heads is especially important in battery-powered disk drive systems, such as disk-drive-based portable audio systems and the like. The power source for this automatic retraction of the read/write heads can be the “back emf” that is generated by the rotation of the disks themselves. Another common approach for retracting heads implements a capacitor across the voice coil motor output, which stores sufficient charge during operation to power the retraction operation upon loss of power. Conventional retract circuitry in modern disk drives control the energy applied to the voice coil motor so that the actuator has sufficient drive energy to climb the ramp at the parking position, without excess velocity that could cause the actuator to rebound from the crash stop.
FIG. 1 illustrates a conventional capacitor-based retract circuit, as implemented in voice coil motor control 210 and as applied to voice coil motor M in a conventional modern disk drive. Voice coil motor control 210 includes the necessary and appropriate conventional circuitry for driving voice coil motor M to position an attached actuator arm, such circuitry including normal drive circuitry 205, which drives motor M via lines VCMB and VCMA. Normal drive circuitry 205 thus includes conventional circuitry for receiving a torque or position signal, and output circuitry, arranged for example as an H-bridge or as a single-ended drive, to apply the appropriate current (of either polarity) to motor M. Sense resistor R_s is connected in series with motor M, such that sensing of the voltage at nodes RSENP, RSENN by feedback control circuitry (not shown) within voice coil motor control 210 can be performed.
The actuator retract function in this conventional arrangement is based on a voltage stored at capacitor 200, which is a relatively large capacitor (e.g., 220 μF), and which is therefore conventionally realized externally to voice coil motor control 10 (which itself is typically contained within an integrated circuit). The source-drain path of n-channel metal-oxide-semiconductor (MOS) transistor 206 selectively connects capacitor 200 to motor M, at node RSENP, in response to a control signal applied to the gate of transistor 206 by retract control logic 204 on line RETCTLN. Retract control logic 204 includes the appropriate conventional circuitry for determine whether the actuator is to be retracted, and for controlling the duration of the retraction event as well as the drive applied to motor M in that event.
As such, as shown in FIG. 1, retract control logic 204 issues a digital signal on lines DIG_RV_SEL to current DAC (digital-to-analog converter) 212, which controls the bias of the retraction drive applied to motor M from capacitor 200, by defining the current I_retr. Voice coil motor M is connected, at node VCMA, to the drain of n-channel MOS transistor 208, which has its source at ground and its gate controlled by low side drive amplifier 209. Low side drive amplifier 209 receives the voltage at node VCMA at a negative input, and a voltage generated by resistor R_retr at a positive input. Resistor R_etr is connected between the source of transistor 206, at node RSENP, and this positive input of low side drive amplifier 209. This voltage is also connected to current DAC 212, and to one leg of current mirror 214; current DAC 212 conducts a current selected by retract control logic 204 on lines DIG_RV_SEL. Another leg of current mirror 214 is connected through n-channel MOS transistor 215 to ground via external resistor R_bias. The gate of transistor 215 is controlled by differential amplifier 216, which receives a reference voltage from retract voltage bandgap circuit 218 at one input, and which receives the voltage at the source of transistor 215 at its other input.
In a retraction event, such as loss of power, retract control logic 204 senses the event and turns on transistor 206; normal driver circuitry 205 is disabled. Current is then conducted from capacitor 200, through transistor 206, and into motor M to apply a torque such that the connected actuator arm is moved toward the parking position. This drive is controlled by the circuitry of voice coil motor control 210, as will now be described.
According to this conventional circuit, the voltage at motor M, specifically at node RSENP, is regulated by retract control logic 204 to equal the voltage on line RETCTLN output, less the gate-to-source voltage (Vgs) of transistor 206. This regulated level, which is based on the output of voltage regulator 207, can remain relatively stable until the voltage across capacitor 200 is discharged, through transistor 206, to a voltage at the voltage on line RETCTLN; in this conventional arrangement, the voltage on line RETCTLN, as regulated by voltage regulator 207, is based on the Vdd power supply voltage, and as such will fall as Vdd falls in a loss of power event. During the retraction period, however, the voltage at node RSENP will remain essentially constant.
The sink current I_retr through resistor R_retr is controlled by the operation of current DAC 212, as will now be explained. The current through transistor 215 is based on the output of retract voltage bandgap circuit 218, but also on the resistance of resistor R_bias, which is reflected at the gate of transistor 215 via differential amplifier 216. This current is mirrored into current DAC 212, and in combination with the digital control signal on lines DIG_RV_SEL, is reflected in the sink current I_retr. As is evident from FIG. 1, in this conventional arrangement, the absolute level of sink current I_retr is thus determined by the resistance of external resistor R_bias. In addition, the operation of transistor 208 and low side drive amplifier 209 ensures that the voltage difference between nodes RSENP and VCMA, which is the voltage across voice coil motor M, is maintained at the level determined by the product of the resistance of resistor R_retr and the current I_retr, until the voltage at capacitor 200 falls to a level below the product of the resistance of resistor R_retr and the current I_retr. The current into voice coil motor M is thus the current sourced from capacitor 200, less this sink current I_retr.
As a result of this arrangement, in this conventional circuit, the retract reference sink current I_retr thus varies inversely with the resistance R_bias, such that the constant voltage (R_retr x I_retr) across motor M depends on the ratio of the resistance R_bias to the resistance R_retr. The voltage to be regulated across motor M is therefore determined by the relative resistance of these large external resistors R_bias, R_retr (e.g., 1.2 MΩ and 1.0 MΩ, respectively). While these resistances can be set with the desired precision, these resistances are hard-wired values. This hard-wiring of the voltage and current drive of motor M for retraction events is thus quite inflexible to the system designer and user.
By way of further background, our commonly assigned copending U.S. patent application Ser. No. 11/323,800, filed Dec. 30, 2005, entitled “Wave Torque retract of disk drive actuator”, and incorporated herein by this reference, describes a control arrangement for the retraction of a disk drive actuator.